1. Field of the Invention
This invention relates to an apparatus for measuring the thickness and/or determining the flaking state of a specimen along its depth by utilizing an ultrasonic wave.
2. Description of the Related Art
Apparatuses using an ultrasonic wave for measurement of various dimensions are already known. An apparatus of this type applies an ultrasonic wave to a specimen, converts the wave (echo) reflected by the specimen into an electric signal, and extracts the component of the electric signal representing the reflected wave from the specimen to determine the condition of the specimen along its depth.
The condition of the specimen determined by such an apparatus normally includes the thickness and/or the state of separation of parts of the specimen.
Now, a known apparatus for determining the flaking state of a specimen along its depth will be described by referring to FIGS. 14, 15A and 15B of the accompanying drawings.
The apparatus comprises a transmitter 51 that generates a single pulse signal at a time. The generated pulse signal is then converted into an ultrasonic wave by a piezoelectric transducer 52, which ultrasonic wave is focused to a minute spot by an acoustic lens 53. A specimen 55 is placed near the focal point of the ultrasonic wave.
Said specimen 55 is mounted on an object stage 56 and the space between the acoustic lens 53 and the specimen 55 is filled with a coupler liquid 54 that transmits the ultrasonic wave.
The ultrasonic wave incident on the specimen 55 is reflected as a function of the acoustic characteristics of the front and back surfaces and the inside of the specimen. The reflected wave then passes the coupler liquid 54, the acoustic lens 53 and is converted into an electric signal by the transducer 52 before it is led to a preamplifier 57.
The output of the preamplifier 57 comprises a number of components that reflect various phenomena involved in the application of the ultrasonic wave including radiation, in-lens reflection, reflection by the front surface of the specimen, reflection inside the specimen and reflection by the back surface of the specimen. The electric signal from the preamplifier 57 containing these components is input to a gate circuit 58. The gate circuit 58 extracts a specific component from those components, and outputs a reflected component signal.
The produced reflected component signals are respectively given to a +detector 59 and a -detector 60. Detector 59 detect a peak level of the entered signal to determine the positive peak intensity of the reflected component signal, while the -detector 60 detects an inverted peak level of the entered signal to determine the negative peak intensity of the signal. The signals detected by the +detector 59 and the -detector 60 are then sent to a comparator 63.
A reflected component signal obtained from a spot of the specimen where IC chips and/or other parts are firmly held together typically shows a waveform as shown in FIG. 15A. When the value of the positive peak is Va.sup.+ and the absolute value of the negative peak is Va.sup.-, they always hold a relationship as expressed below. EQU Va.sup.+ &lt;Va.sup.- ( 1)
On the other hand, a reflected component signal corresponding to a spot where separation of parts exists shows a waveform shown in FIG. 15B and having a phase shifted by .pi. from that of the waveform for a firm spot (without separation of parts) as shown in FIG. 15A. When the value of the positive peak is Vb.sup.+ and the absolute value of the negative peak value is Vb.sup.-, the two values show a relationship as expressed below. EQU Vb.sup.+ &gt;Vb.sup.- ( 2)
The comparator 63 transmits "1" to a computer 64 when the value for the positive peak is greater than the absolute value for the negative peak, whereas it sends out "0" when the former is smaller than the latter. Output signals from the +detector 59 and those from the -detector 60 are converted into digital signals by respective A/D converters 61a and 61b before being stored in a memory 62.
The data obtained by way of the above operation provide information only for an examined spot in the specimen 55. An XY-scanner 67 scans the specimen 55 on an XY plane by moving the acoustic lens 53 relative to the specimen to bring forth two-dimensional visual data for the specimen, telling where, if any, separated parts are found.
The two-dimensional visual data stored in the memory 62 are then processed by the computer 64, which displays an image of the specimen, emphatically coloring those parts it has judged to be loose and separated from the rest of the specimen in a specific way to make them easily noticeable.
FIG. 16 illustrates a known apparatus for measuring the thickness of a specimen by an ultrasonic wave.
This apparatus comprises a variable frequency oscillator 71 and an ultrasonic probe 72, the output voltage of the oscillator 71 being applied to the ultrasonic probe 72 that performs an electro-acoustic conversion to produce an ultrasonic wave out of the applied voltage. The ultrasonic wave produced by the ultrasonic wave is then applied to a specimen 74 throw a coupler liquid 73.
The wave reflected by the specimen 74 is then brought back into the ultrasonic probe 72 by way of the coupler liquid 73 and converted by the probe into a voltage representing the intensity of the reflected wave. Consequently, the level of the current in the oscillator 71 is subjected to changes.
Now, as the frequency of oscillation of the oscillator 71 is varied, the wavelength .lambda. of the ultrasonic wave in the specimen changes. When the thickness d of the specimen 74 is equal to the half wavelength multiplied by an integer (n) or EQU n.lambda./2=d, (3)
a stationary wave appears in the specimen 74 and resonates. The energy of oscillation of the ultrasonic wave when the resonance takes place is converted into electricity by the probe 73, which is then added to the electric current in the oscillator 71.
The electric current in the oscillator 71 is amplified by an amplifier 75 and displayed on an oscilloscope 76. Thus, as the frequency of oscillation of the oscillator 71 is varied, a waveform as shown in FIG. 17 will appear on the CRT of the oscilloscope 76, exhibiting regularly separated ridges that indicate frequencies where resonance takes place. If the frequencies for the ridges are expressed by f1, f2 . . . fn, fn+1, an equation as shown below can be obtained by using the formula (3) above. EQU d=nV/2fn (4)
where V is the velocity of sound. The thickness of a specimen 74 can be determined by the equation (4) above, if n and v are known.
Even if n is not known, the thickness of a specimen 74 can be determined by calculating the difference of two neighboring resonance frequencies and using the formula below. EQU d=V/2(fn+1-fn) (5)
An apparatus for measuring the state of a specimen by means of an ultrasonic wave as described above is, however, accompanied by certain disadvantages, which will be explained below.
The ability of any known apparatus for determining the flaking state of parts of a specimen by using an ultrasonic wave is subject to limitations because the relationship between the positive and negative peak values of an echo wave reflected from a spot in the specimen having parts which are loose and separated from one another can vary depending on the frequency of the single pulse signal applied to the specimen, the resonance frequency of the transducer of the apparatus, the acuteness of resonance of the transducer, the absorption coefficient of the specimen and other factors.
If, for instance, a frequency of or near 30 MHz is used with such an apparatus to determine the condition of separation of parts of a specimen following a common practice and if the absorption coefficient of the specimen is relatively small, the equation (1) will hold true for an echo wave reflected from a firm spot where no separation of parts exist, whereas the equation (2) will be valid for an echo wave reflected from an area where separation of parts is found.
Conversely, however, the equation (2) will hold its validity for an echo wave reflected from an area where separation of parts exists and the equation (1) will hold effective for an echo wave reflected from a spot where no separation of parts is existent if a frequency of or near 50 MHz is involved.
To make the matter worse, equation (1) will be effective for an echo wave reflected from a firm spot when the specimen has a large absorption coefficient even if a frequency of or near 50 MHz is used, whereas equation (2) will hold true for an echo wave reflected from a loose spot where separation of parts exists, a phenomenon quite contrary to an occasion where the absorption coefficient is relatively small.
Thus, such a known apparatus for determining the flaking state of parts of a specimen that shows a significantly variable relationship between the positive and negative peak values of an echo wave reflected from a spot of a specimen to be examined depending on the performance of the transducer of the apparatus, the frequency of the ultrasonic wave applied to the specimen, the level of the absorption coefficient of the specimen for the ultrasonic wave involved and other factors as described above can be used only with a limited frequency range and a specific transducer if the state of separation of parts of the specimen needs to be known accurately and, therefore, has only a poor applicability.
Besides, any known apparatus of the above described type requires a long time for determining the thickness of a specimen by an ultrasonic wave and hence is not good for a quick instrumentation.
This is because the apparatus employs a continuous wave and requires the frequency of the wave to be continuously changed to find out a resonance frequency for each measurement.
Moreover, such a known apparatus is not good for measuring the thickness of a specimen having a partial upheaval or a specimen having uneven upper and/or lower surfaces.